Search Results (13)

The West African atmospheric energy budget is assessed for the first time across three contrasting monsoon seasons (dry, wet, and moderate) using the latest version of the Canadian Regional Climate Model (CRCM6/GEM5). The model is driven by ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). A formalism appropriate for regional climate energetics is employed to quantify the primary physical processes occurring during the West African Monsoon, with the aim of highlighting those that exhibit significant inter-seasonal variability. The atmospheric energy path shows that the time-mean available enthalpy (A_M) reservoir, reflecting high surface temperatures and a lapse rate characteristic of a dry atmosphere, dominates other energy reservoirs. A_M is converted into the time-mean kinetic energy (K_M) and the time-variability available enthalpy (A_E) reservoirs, which are converted into a time-variability kinetic energy reservoir (K_E) through barotropic and baroclinic processes. A_E is the lowest energy reservoir, confirming smaller temperature variations in the tropics compared to higher latitudes. Kinetic energy reservoirs K_M and K_E have the same order of magnitude, suggesting that mean flow is as important as eddy activities during the season. The atmospheric energy cycle computed for three contrasting rainy seasons shows that time-variability energy reservoirs (A_E and K_E) and main terms acting upon them, are proportional to the rainfall activity, being higher (lower) during rainy (dry) years. It also reveals that, while C_A (conversion from A_M to A_E) and the generation term G_E feed wave’s development, the frictional term D_E counteracts the generation of K_E to dampen the creation of transient eddies. These findings suggest that the atmospheric energetic formalism could be applied on West African seasonal forecasts and future climate simulations to implement adaptation strategies. Full article

The scale-dependent transport of Saharan dust aerosols by African easterly waves (AEWs) is examined analytically and numerically. The analytical analysis shows that the meridional and vertical wave transports of dust are modulated by the Doppler-shifted frequency, ${\omega }_d$, and the wave growth rate, ${\omega }_i$, both of which are functions of the zonal wave scale. The analytical analysis predicts that the AEW dust transports, which are driven by the Reynolds stresses acting on the mean dust gradients, are largest for the twin limits: ${\omega }_d\to 0$, which corresponds to flow near a critical surface, a local effect; and ${\omega }_i\to 0$, which corresponds to the slowest growing waves, a global effect. The numerical analysis is carried out with the Weather Research and Forecasting (WRF) model, which is radiatively coupled to the dust field. The model simulations are based on an AEW spectrum consistent with observations. The simulations agree with the theoretical predictions: the slowest growing waves have the strongest transports, which are as much as ~40% larger than the transports of the fastest growing wave. Although the transports are highly scale-dependent, largely due to the scale dependence of ${\omega }_i$, the location of the critical surface and thus the location of the maximum dust transports are not. Full article

This study revisited the association of African easterly waves (AEWs) to Atlantic tropical cyclone (TC) development using weather states (WSs) from the International Satellite Cloud Climatology Project, National Hurricane Center best track hurricane data (HURDAT2), and reanalysis products. The WS data are used as a proxy for two different types of deep convection. This study covers July–October 1984–2009. Statistical analysis based on HURDAT2 and objectively tracked AEWs has shown that a small fraction (~20%) of the AEWs that propagate from Africa serve as TC precursors. About 80% of the AEWs from the continent were non-developing. As in the past work, our study showed an important difference between developing and non-developing AEWs. Composites based on developing AEWs revealed well-organized large scale deep convection (one type, composed of mesoscale systems and thick anvil clouds) is tightly coupled to the AEW trough, while scattered, less well-organized deep convection (second type, isolated cumulonimbus and cumulus congestus clouds) dominated a large area downstream of the developing AEW trough. Developing AEWs propagate westwards while strengthening. In contrast, non-developing AEWs showed that the peak well-organized deep convection is located either behind (to the east of) or far ahead (to the west) of the AEW trough (peaks values are not in close proximity). Moreover, well-organized deep convections associated with non-developing AEWs were weaker than those associated with developing AEWs. The results indicated that convective activity ahead of the non-developing AEWs is weak. Positive relative humidity (RH) anomalies dominate the area around AEWs and downstream over the main TC development region. In contrast, negative RH dominated the main TC development region ahead of non-developing AEWs, suggesting an unfavorable environment downstream of the AEWs. The results also showed that developing AEWs maintained stronger features in the lower and middle troposphere, while non-developing AEWs exhibited weaker structures, in agreement with past work. Full article

Theory and modeling are combined to reveal the physical and dynamical processes that control Saharan dust transport by amplifying African easterly waves (AEWs). Two cases are examined: active transport, in which the dust is radiatively coupled to the circulation; passive transport, in which the dust is radiatively decoupled from the circulation. The theory is built around a dust conservation equation for dust-coupled AEWs in zonal-mean African easterly jets. The theory predicts that, for both the passive and active cases, the dust transports will be largest where the zonal-mean dust gradients are maximized on an AEW critical surface. Whether the dust transports are largest for the radiatively passive or radiatively active case depends on the growth rate of the AEWs, which is modulated by the dust heating. The theoretical predictions are confirmed via experiments carried out with the Weather Research and Forecasting model, which is coupled to a dust conservation equation. The experiments show that the meridional dust transports dominate in the passive case, while the vertical dust transports dominate in the active case. Full article

Current studies report inconsistent results about the impacts of Saharan dust on the development of African Easterly Waves (AEWs), the African Easterly Jet (AEJ), and tropical cyclones (TCs). We present a modeling case study to further elucidate the direct radiative impacts of dust on the early development stage of a TC. We conducted experiments using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem-V3.9.1) to simulate Hurricane Earl (2010) which was influenced by the dusty Saharan Air Layer (SAL). We used the aerosol product from ECMWF MACC-II as the initial and boundary conditions to represent aerosol distribution, along with typical model treatment of its radiative and microphysical effects in WRF. Our simulations at 36-km resolution show that, within the first 36 h, the presence of dust weakens the low-pressure system over North Africa by less than 1 hPa and reduces its mean temperature by 0.03 K. Dust weakens and intensifies the AEJ at its core and periphery, respectively, with magnitudes less than 0.2 m/s. Dust slightly shifts the position of 600 hPa AEW to the south and reduces its intensity prior to impacting the TC. Finally, TC with dust remains weaker. Full article

The aerosol vertical distribution in the tropical cyclone (TC) main development region (MDR) during the recent active hurricane seasons (2015–2018) was investigated using observations from NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Satellite. The Total Attenuated Backscatter (TAB) at 532 nm was measured by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP Lidar) onboard CALIPSO which is a polar orbiting satellite that evaluates the role clouds and atmospheric aerosols play in Earth’s weather, climate and air quality. The TAB was used to illustrate the dispersion and magnitude of the aerosol vertical distribution in the TC-genesis region. A combination of extinction quality flag, cloud fraction, and cloud-aerosol discrimination (CAD) scores were used to filter out the impact of clouds. To better describe the qualitative and quantitative difference of aerosol along the paths of African Easterly Waves (AEWs), the MDR was further divided into two domains from 18° W to 30° W (Domain 1) and 30° W to 45° W (Domain 2), respectively. The distribution of average aerosol concentration from the time of active cyclogenesis was compared and quantified between each case. The resulting observations suggest that there are two distinct layers of aerosols in the vertical profile, a near surface layer from 0.5–1.75 km and an upper layer at 1.75–5 km in altitude. A quantification of the total aerosol concentration values indicate domain 2 cases were associated with higher aerosol concentrations than domain 1 cases. The environmental variables such as sea surface temperature (SST), vertical windshear (VWS), and relative humidity (RH) tended to be favorable for genesis to occur. Among all cases in this study, the results suggested tropical cyclone genesis and further development occurred under dust-loaded conditions while the environmental variables were favorable, indicating that dust aerosols may not play a significant role in inhibiting the genesis process of TCs. Full article

This study highlights the influence of convectively coupled Kelvin wave (KW) activity on deep convection and African easterly waves (AEWs) over North Africa during dry and wet boreal summer rainfall years. Composite analysis based on 25 years of rainfall, satellite observed cold cloud temperature, and reanalysis data sets show that KWs are more frequent and stronger in dry Central African years compared with wet years. Deep convection associated with KWs is slightly more amplified in dry years compared with wet years. Further, KW activity over North Africa strengthens the lower level zonal flow and deepens the zonal moisture flux in dry years compared with wet years. Results also show that enhanced KW convection is in phase with above-average AEW variance in dry years. However, enhanced KW convection is out-of-phase with average AEW activity in wet years. In general, this study suggests that KW passage over Africa enhances convective activity and more strongly modulates the monsoon flow and moisture flux during the dry years than wet years. Full article

The present study utilizes three high-resolution simulations from the Regional Climate Model version 4 (RegCM4) to examine the late 21st century changes (2080–2099) in the West African Monsoon (WAM) features. A set of three Earth System Models are utilized to provide initial and lateral boundary conditions to the RegCM4 experiments. Our analysis focuses on seasonal mean changes in WAM large-scale dynamical features, along with their connections with the summer monsoon precipitation. In the historical period, the simulation ensemble means mimic reasonably well the intensity and spatial distribution of the WAM rainfall as well as the WAM circulation patterns at different scales. The future projection of the WAM climate exhibits warming over the whole West Africa leading to precipitation reduction over the Sahel region, and a slight increase over some areas of the Guinea Coast. The position of the African Easterly Jet (AEJ) is shifted southward and the African Easterly Waves (AEWs) activities are reduced, which affect in turn the WAM rainbelt characteristics in terms of position and strength. Overall the changes in simulated AEJ and AEWs contribute substantially to reduce the seasonal summer mean precipitation in West Africa by the late 21st century, with prevailing negative changes in the Savanna-Sahel region. To further explore the robustness of the relationships revealed in this paper, future studies using different high-resolution regional climate models with large ensemble are recommended. Full article

An analytical analysis is combined with numerical modeling simulations in order to expose the physical and dynamical processes that control the zonal-mean transport of Saharan mineral dust aerosols during the incipient growth phase of African easterly waves. The analytical analysis provides the theoretical basis for understanding and predicting how the waves and background flow combine to affect the zonal-mean eddy transports of dust. The analytically derived transport equations―which are valid for any wave field, irrespective of its spatial or temporal scale―predict that the eddy transports of dust are largest where the maximum in the background dust gradients coincide with a critical surface, i.e., where the Doppler-shifted frequency of the wave field vanishes. Linear simulations of the eddy dust transports are conducted using a mechanistic version of the Weather Research and Forecasting (WRF) model coupled to an interactive dust model. The simulations show that the eddy dust transports are directed down the background dust gradients and that the meridional transports of dust dominate over the vertical transports. The numerical simulations confirm the theoretical predictions. The predictions are used to explain recent statistical analyses of reanalysis data for dust-coupled African easterly waves. Full article

Recent advances in computational and global modeling technology have provided the potential to improve weather predictions at extended-range scales. In earlier studies by the author and his coauthors, realistic 30-day simulations of multiple African easterly waves (AEWs) and an averaged African easterly jet (AEJ) were obtained. The formation of hurricane Helene (2006) was also realistically simulated from Day 22 to Day 30. In this study, such extended predictability was further analyzed based on recent understandings of chaos and instability within Lorenz models and the generalized Lorenz model. The analysis suggested that a statement of the theoretical predictability of two weeks is not universal. New insight into chaotic and non-chaotic processes revealed by the generalized Lorenz model (GLM) indicated the potential for extending prediction lead times. Two major features within the GLM included: (1) three types of attractors (that also appeared in the original Lorenz model) and (2) two kinds of attractor coexistence. The features suggest a refined view on the nature of weather, as follows: The entirety of weather is a superset that consists of chaotic and non-chaotic processes. Better predictability can be obtained for stable, steady-state solutions and nonlinear periodic solutions that occur at small and large Rayleigh parameters, respectively. By comparison, chaotic solutions appear only at moderate Rayleigh parameters. Errors associated with dissipative small-scale processes do not necessarily contaminate the simulations of large scale processes. Based on the nonlinear periodic solutions (also known as limit cycle solutions), here, we propose a hypothetical mechanism for the recurrence (or periodicity) of successive AEWs. The insensitivity of limit cycles to initial conditions implies that AEW simulations with strong heating and balanced nonlinearity could be more predictable. Based on the hypothetical mechanism, the possibility of extending prediction lead times at extended range scales is discussed. Future work will include refining the model to better examine the validity of the mechanism to explain the recurrence of multiple AEWs. Full article

Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15–20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa. Full article

We used the Abdu Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model version 4.5 (RegCM4.5), to investigate the potential impacts of land cover change of the Sahel–Sahara interface on the West African climate over an interannual timescale from 1990 to 2009. A simulation at 50 km grid spacing is performed with the standard version of the RegCM4.5 model (control run), followed by three vegetation change experiments at the Sahel-Sahara interface (15° N and 20° N): forest, tall grass, and short grass savanna. The impacts of land cover change are assessed by analyzing the difference between the altered runs and the control one in different sub-domains (western Sahel, central Sahel, eastern Sahel, and Guinea). Results show that the presence of forest, tall grass, and short grass savanna at the Sahel–Sahara interface tends to decrease the mean summer surface temperature in the whole domain. Nevertheless, this decrease is more pronounced over the Central Sahel when considering the forest experiment. This temperature decrease is associated with a weakening (strengthening) of the sensible (latent) heat flux in the whole domain. An analysis of the radiation field is performed to better explain the changes noted in the latent heat flux, the sensible heat flux, and the surface temperature. When considering the rainfall signal, the analysis shows that the afforestation options tend to alter the precipitation in the considered sub-domains substantially by increasing it in the whole Sahel region, with strong interannual variability. This rainfall increase is associated with an increase of the atmospheric moisture. Finally, we investigated the impacts of the afforestation options on some features of the rainfall events, and on the atmospheric dynamics during wet and dry years. All afforestation options tend to increase the frequency of the number of rainy days in regions located south of 18° N during both periods. Nevertheless, this increase is stronger over the Central and Eastern Sahel during wet years in the forest case. All afforestation experiments induce an increase (decrease) of the low-levels monsoon flux in the Eastern Sahel (western Sahel) during both periods. At the mid-levels, the three afforestation options tend to move northward and to decrease the intensity of the African Easterly Jet (AEJ) south of 13° N during wet and dry years.The intensity of the AEJ is weaker during the wet period. The vegetation change experiments also affect the Tropical Easterly Jet (TEJ), especially during wet years, by increasing its intensity over the southern Sahel. The analysis of the activity of African Easterly Waves (AEWs) patterns exhibits a decrease of the intensity of these disturbances over the Sahel during both periods. This may be due to the weakening of the meridional temperature contrast between the continent and the Gulf of Guinea due to the Sahel–Sahara surface temperature cooling induced by the afforestation. In summary, this study shows that during both periods, the increase of the atmospheric moisture due to the afforestation is associated with favorable AEJ/TEJ configurations (weaker and northward position of the AEJ; stronger TEJ) which in turn may create a stronger convection and then, an increase in the Sahel rainfall. This Sahel rainfall increase is associated with a strengthening of the intense and heavy rainfall events which may impact diversely local populations. Full article

Spatiotemporal conditions that rule hydro-climatology over northern South America and the Caribbean Sea are influenced by a large amount of phenomena taking place at different timescales. Characterizing the activity of the AEWs over northern South America and the Caribbean is an imperative work to do in order to improve our understanding of the tropical atmospheric dynamics involved in hydrology and climate features over the region. The latter regulates the availability of very important resources such as water. Furthermore, AEWs activity plays an important role on air quality characteristics as a consequence of its connections with dust transport. In order to approach an adequate characterization of the AEWs activity over the region, this work addresses the relationship between these atmospheric perturbations and the occurrence or inhibition of precipitation, as well as possible connections with dust transport, when the AEW’s oscillations take place over northern South America and the Caribbean region. In particular, relative vorticity and outgoing long-wave radiation are used to identify AEW’s activity during the 1983–2013 period, together with daily precipitation anomalies, surface divergence, vertical integrated moisture flux, and Aerosol Optical Depth, in order to understand how the passage of AEWs could influence meteorological interactions in the region. Full article